This invention relates to apparatus for and method of analysing a mixture comprising a fluid and a plurality of electrically charged particles contained therein.
Particle concentrations in aerosols (a suspension of particles in a gas) are often measured by electrostatic techniques based on the principle of charging the particles in a sample of the aerosol and collecting them on one or several collection elements such as electrodes or filters. The current flowing to these electrodes or filters, here referred to as xe2x80x9ccollection electrodesxe2x80x9d, is measured and indicates the quantity of particles collected and hence their concentration in the aerosol.
The particles may be charged by any one of a number of methods, such as ultraviolet irradiation or corona discharge; or natural charging (often associated with a combustion process) may be relied upon.
Frequently, differences in mobility (the readiness of particles to diff-use or drift through the gas) are used to separate different sizes of particles before collecting them on the collection electrodes. Some devices alternatively use differences in momentum for this discrimination.
Such devices are used to make measurements of the number of particles and sometimes the spectrum of particle sizes in aerosols, but are limited to resolving accurately only relatively slow changes in the particle concentration. This is because faster changes lead to transient discrepancies between the actual particle concentration and the measured current which are caused by the rate of change of the concentration of charged particles in the aerosol near the detectors.
According to the present invention, modifications are made to the design of electrostatic particle measurement instruments to compensate for or eliminate the transient currents produced by the rate of change of charge near the sensing electrodes, and hence reduce the transient errors in measured particle concentrations.
According to a first aspect of the invention, this is achieved by apparatus for analysing a mixture comprising a fluid and a plurality of electrically charged particles therein, the apparatus comprising a collection element for collecting said particles and providing an output relating to the number of particles incident thereon, and compensation electrode means, spaced from the collection element, which is responsive to charged particles which pass in the vicinity of, but which are not collected by, the collection element, thereby to enable the output from the collection element to be used to determine the charge collected by the collection element.
Thus, by providing compensation electrode means the invention enables spurious measurements caused by particles which induce a current in the collection element, but are not themselves collected by that element, to be avoided. The collection element may comprise any suitable element for collecting charged particles so that the total charge or current resulting from the collection of charged particles can be measured.
For example, the apparatus may comprise an electrostatic low pressure impactor (ELPI) instrument which charges the particles in an aerosol to be measured and then passes the aerosol through a column of impactors. The impactors comprise perforated plates followed by collection plates which may be covered with grease. When the aerosol passes through the perforations, relatively massive particles are forced by their momentum to hit the collection plates where they are detected whereas the lighter particles are carried by the gas flow to the subsequent stages. The size of the perforations, size of the plates and the operating pressure are varied throughout the column such that the largest particles are detected on the earliest collection plates and successively smaller particles are detected on later collection plates. Measurement of the electrical current flowing to these collection plates indicates the number of particles detected by each and hence the concentration of size class of particles. Each collection plate thus functions as an electrode.
Alternatively, the apparatus may have a collection element which comprises an electrode to which is applied an accelerating voltage for attracting the charged particles. The apparatus may have a succession of such electrodes arranged along a conduit so as to provide an output representative of the size spectrum (i.e. the concentration of particles in each of a number of size classes).
In such a case, the compensation electrode means may to advantage comprise a shielding electrode which overlies each of the collection electrodes, the arrangement being such that charged particles on the other side of the shielding electrode from the collection electrodes are prevented or inhibited by the shielding electrode from inducing currents on the collection electrodes.
The shielding electrode may to an advantage comprise a conductive grid. Preferably, the conduit is cylindrical, each collection electrode is annular and is coaxial with said cylinder and the shielding electrode is also cylindrical and coaxial with the conduit.
Instead of shielding the collection electrode, the compensation electrode means may alternatively be so arranged as to provide an output which can be processed to provide a correction signal for removing or inhibiting components of the output from the collection electrode caused by induced current.
Such a compensation electrode means can be controlled so as to be maintained at a voltage which results in a collection of no significant number of particles. Alternatively, the compensation electrode means may collect all of the particles which pass in the vicinity of the collection electrode without being collected by the latter.
In arrangements which have a plurality of collection electrodes distributed along a conduit, the compensation electrode means for a given collection electrode may be constituted by all of the collection electrodes positioned downstream thereof. If the downstream collection electrodes between them, collect all of the particles which are not collected by the first said collection electrode, their outputs can be used to obtain an indication of the induced current on said collection electrode.
According to a second aspect of the invention, in a method of measuring the current flow to a collection electrode to indicate the quantity of charged particles in an aerosol, there is an improvement comprising the step of eliminating or compensating for the part of the current flowing to said collection electrode which is caused by the rate of change of charged density in said aerosol near said collection electrode.